When preparing OE devices, especially OLED devices, it is essential to get even film formation, as a 3% variation in film thickness is visible to the naked eye. This is commonly very difficult to achieve. Frequently poor uniformity is seen, this can take the form of coffee staining where the ink is thicker at the edges and thinner in the centre; reticulation where the ink coalesces into large droplets; point defects where small holes appear within the surface of the film. In dual solvent systems Marangoni effects can also been seen.
US 2007/0221885 relates to a three solvent system and is specifically relating to one solvent used to solubilise the active material, a second solvent that has a similar surface energy to the substrate and a third solvent which is used to make the other two solvents miscible. 3,4-dimethylanisole is mentioned within a list of solvents as a potential material to be used, it is not exemplified as one of the solvents. The main drive for this patent is to obtain printability onto low surface energy substrates.
WO 2006/122732 relates to compositions, especially solutions, of at least one organic semiconductor emitting light from the triplet state, in an organic solvent or solvent mixture. The solvent may include 3,4-dimethylanisole. However, no example is provided.
Furthermore WO 2011/076325 A1 describes the use of dimethylanisole as a solvent. However, the solvent is used for small molecules having a molecular weight of at most 5,000 g/mol. No hints regarding OE devices which comprise polymers as OSC materials are provided.
The prior art provides compositions being useful in order to process organic light emitting and charge transporting materials. However, it is a permanent desire to improve the performance of the OE layer, preferably the OLED layer, such as efficiency, lifetime and sensitivity regarding oxidation or water.
In addition thereto, the formulation should enable a low-cost and easy printing process. The printing process should allow a high quality printing at high speed.
It is therefore desirable to have improved formulations comprising an OSC that are suitable for the preparation of OE devices, especially thin film transistors, diodes, OLED displays and OPV cells, which allow the manufacture of high efficient OE devices having a high performance, a long lifetime and a low sensitivity against water and/or oxidation. One aim of the present invention is to provide such improved formulations. Another aim is to provide improved methods of preparing an OE device from such formulations. Another aim is to provide improved OE devices obtained from such formulations and methods. Further aims are immediately evident to the person skilled in the art from the following description.
Surprisingly it has been found that these aims can be achieved, and the above-mentioned problems can be solved, by providing methods, materials and devices as claimed in the present invention, especially by providing a process for preparing an OE device using a formulation of the present invention.